The present invention relates generally to touch screen systems and touch screen controllers, and more particularly to circuits and methods for reducing/eliminating noise, especially noise associated with the turning on and off of LCDs (liquid Crystal displays) of touch screen assemblies that are controlled by the touch screen controllers.
The closest prior art is believed to include (1) U.S. Pat. No. 6,661,408 entitled “Touch Screen Capable of Isolating Noise Signals” issued Dec. 9, 2003 to Chen, (2) the Analog Devices, Inc. AD7877 touch screen controller specification sheet, (3) commonly assigned U.S. Pat. No. 6,246,394 entitled “Touch Screen Measurement Circuit and Method” issued Jun. 12, 2001 to Kalthoff et al., incorporated herein by reference, and (4) commonly assigned U.S. Pat. No. 6,738,048 entitled “Touch Screen Controller” issued May 18, 2004 to Rundel, also incorporated herein by reference.
“Prior Art” FIG. 1 herein is similar to FIG. 1 of above mentioned U.S. Pat. No. 6,246,394, which discloses a touch screen digitizing system that includes a touch screen unit or assembly 30,31 including a first resistive screen 30 with opposed x+ and x− terminals, a second resistive screen 31 with opposed y+ and y− terminals, and an ADC 22. The various terminals of touch screen assembly 30,31 are connected to corresponding terminals of a touch screen controller (TSC) chip 1A including a first switch 19 which is coupled between a first reference voltage (ground) and the x− terminal, and a second switch 18 which is coupled between the x+ terminal and a second reference voltage +VCC for energizing the first resistive screen 30. A third switch 21 is coupled between ground and the y− terminal, and a fourth switch 20 is coupled between the y+ terminal and VCC for energizing the second resistive screen 31. Switching circuitry 15,17 couples an input of the ADC 22 to the y+ terminal while the first resistive screen 30 is energized and the second resistive screen 31 is not energized, and also couples the input to the x+ terminal while the second resistive screen 31 is energized and the first resistive screen 30 is not energized. More specifically, the various terminals of the resistive screens 30 and 31 are connected to the drains of the various corresponding driver transistors 18, 19, 20 and 21. Control circuit 41 controls the various driver transistors, switches, and ADC 22 by means of conductors 42, and also includes a data register 48 which receives/updates the analog to digital conversion results from ADC 22 so they are available to be read by a host processor 3 via a control/data bus 40. Control circuit 41 also can generate a processor interrupt request signal IRQ on conductor 47. Touch screen controller 1A of FIG. 1 also includes a conventional pen-touch detection circuit (not shown).
A typical PDA (personal digital assistant) or a tablet PC may have a power-saving mode that would turn a signal-indicating LCD (liquid crystal to splay) on and off frequently, and that typically generates a considerable amount of noise that can corrupt digitized touch point coordinate data being produced at the same time.
The two resistive layers of a touch screen assembly normally are placed over and in close proximity to an LCD screen. Therefore, noise can be coupled from the screen onto the resistive layers, causing errors in the measurements of the touch point coordinates. For example, a jitter might be noticeable in the one-screen cursor position. In most LCD touch screen systems, a signal such as an LCD inverts signal or other control signal is present, in the noise usually is coupled onto the touch screen during the active period of this signal. The touch point coordinate data and the noise data both are collected and averaged, and the averaged data includes an offset due to the LCD switching noise.
FIG. 4 of above-mentioned U.S. Pat. No. 6,661,408 to Chen shows a ring-shaped antenna 6 on the bottom of the touch screen assembly which detects noise signals associated with turning on and off of a signal-indicating LCD (liquid crystal display) which indicates if the touch screen sensor is activated. The antenna ring 6 detects the LCD switching noise and produces a signal which is used to remove or cancel the LCD noise from the data. A disadvantage of this prior art is the extra cost of the conductors associated with the antenna 6.
In the Analog Devices AD7877 touch screen controller, it is only during the sample or acquisition phase of the ADC operation that noise from the LCD screen has an effect on the ADC measurements. During the hold or conversion phase, the noise has no effect, because the voltage of the input of the ADC has already been acquired. Therefore, to minimize the effect of noise on the touch screen measurements, the ADC acquisition phase is halted. The LCD control signal should be applied to the STOPACQ pin. To ensure that the ADC acquisition phase never occurs during the noisy period when the LCD signal is active, the AD7877 monitors this signal, and no ADC acquisitions take place when the LCD control signal is active. Any ADC acquisition that is in progress when the LCD signal becomes active is aborted and restarts when the signal becomes inactive again. A disadvantage of this approach is the additional cost of connection of the touch screen controller to the host processor and the additional software burden and synchronization requirements on the host controller.
Even-Odd Transposition Algorithm sorting algorithms are known and disclosed in various references, including the text “The Art of Computer Programming—Searching and Sorting” by D. L. Knuth, Addison-Wesley, 1973. In Prior Art FIG. 2, L is the name of an array L[0:n−1] of n numbers that need to be sorted in accordance with their respective values. The number n is an even integer which represents the size of the array L. “p” is the number of such arrays of numbers that need to be sorted.
Blocks 45, 46, and 47 perform the function of defining the indexing parameters for the loops in the sorting algorithm. For an implementation in which multiple processes are sequentially performed as indicated in the flowchart of FIG. 3B, decision block 48 determines which process is to be performed next, until all processes have been completed, in which case the algorithm stops as indicated by label 49. If the sorting process has not been completed, and affirmative determination is made in decision block 48, and as indicated in block 50, index I is set equal to 0 to establish the procedure of comparing the value of each input word associated with an even-numbered value of index “I” with the value of the input word having the next higher odd-numbered value of index “I”. In every such comparison, if the smaller value input word is not already in the lower position in the array L, its position in the array is automatically swapped with the other input word. For example, if the comparison of block 52 determines that the value of input word L[i=2] is lower than the value of input word L[i=3], their positions will be swapped, as indicated in block 53 (wherein “tmp” is a temporary variable). Otherwise their positions will be unchanged, and the sorting algorithm goes from decision block 52 in which the comparison is made to block 54, wherein the index I is incremented by 2. Then the sorting algorithm returns to decision block 51 and compares the incremented index I with the array size number n in block 51 and compares the values of the next lower pair of even index and odd index input words L[i=4] and L[i=5]. The process is repeated until the input word having the highest value is in the top position of the sorted array and the input word having lowest value is located in the bottom position of the sorted array.
For a sequential implementation in which multiple sub-arrays are processed one at a time, when a negative determination is made in decision block 51, the compare and swap process of blocks 51-54 is repeated in blocks 56-60 until a negative determination is obtained in decision block 48. The sorting then is complete, as indicated by “stop” label 49.
There is an unmet need for lower cost touch screen controller circuitry and techniques for reducing/eliminating noise.
There also is an unmet need for lower cost touch screen controller circuitry and techniques for reducing/eliminating noise, such as noise due to switching of an LCD of a touch screen system.
There also is an unmet need for lower cost touch screen controller circuitry and techniques for reducing/eliminating Gaussian noise, such as Gaussian noise in the resistance of the pair of resistive sheets in a touch screen assembly.